@@Mort-lemur   Question 1: I was wondering when you add a new order_total field (ot_custom) does it calculate the taxes correctly?   Question 2: And can you add a new ot_custom field in AJAX mode? I couldn't. Because when I enter the title I have to click "tab-key" or click with my mouse cursor on the amount field but just at that moment the page updates (AJAX) and it disappears.   If your answer for question 1 is "no" then look at this following fix. (from bugfunder, 23. Jan. 2011, http://addons.oscommerce.com/info/1435) This fix has to be applied to both files edit_orders.php and edit_orders_ajax.php   Look for this
  $j=1; //giving something a sort order of 0 ain't my bag baby $new_order_totals = array(); add right after it
  //BOF ot_custom tax fix   $tax_to_add=0; //EOF ot_custom tax fix Look for this
  if ( ( ($order_totals[$i]['code'] == 'ot_tax') && ($order_totals[$i]['code'] == $ot_class) && ($order_totals[$i]['title'] == $ot_title) ) || ( ($order_totals[$i]['code'] != 'ot_tax') && ($order_totals[$i]['code'] == $ot_class) ) ) { //6 //only good for components that show up in the $order_totals array add right after it
  //BOF ot_custom tax fix               if ($order_totals[$i]['code'] == 'ot_tax') {       $order_totals[$i]['value']+=$tax_to_add;       $order_totals[$i]['text']=$currencies->format($order_totals[$i]['value'], true, $order->info['currency'], $order->info['currency_value']);   } //EOF ot_custom tax fix Look for this
  } elseif ( (tep_not_null($ot_value)) && (tep_not_null($ot_title)) ) { // this modifies if (!strstr($ot_class, 'ot_custom')) { //3 add right after it
  //BOF ot_custom tax fix //This calculates tax on ot_custom    $tax_to_add += $ot_value * $tax / (100 + $tax); //EOF ot_custom tax fix Look for this
      $j++;   } //end 7 } //end 2 and add right "above" it
  //BOF ot_custom tax fix              //This calculates tax on non-standard    $tax_to_add += $ot_value * $tax / (100 + $tax);    $order->info['total'] += $ot_value; //EOF ot_custom tax fix     This worked for me so far. I have only one tax class in my shop therefore i can't tell if that fix would work on a shop with multiple tax classes. And since you use a different version then i do it could be that this fix is already applied in your files and therefore not necessary anymore.